Most commercial latexes are classified as anionic. This means that there is a negative charge on the latex particle. This negative charge can be produced in several ways: (1) use of anionic monomers such as carboxylic or sulfonic acids or their salts; (2) the normal incorporation of anionic initiator fragments derived from persulfates; and (3) adsorption of the anionic surfactants used to generate latex particles and stabilize their growth. Of course, like all salts there is an oppositely charged counterion that is relatively free in the water phase to keep the overall charge balanced.
The negative charge on the latex particle plays a crucial part in its keeping the latex stable. Electrostatic repulsion of the like (-) charges keep the particles from clumping together and forming larger clusters that eventually precipitate from the water phase.
Any variable that reduces the effective surface charge decreases the latex stability. Hence, adding simple salts to a latex can destabilize it. The cationic portion of a simple salt associates with the negative charges on the latex and reduces the overall charge at the particle surface. The effect of the cationic counterion depends upon both its concentration and its charge or valency. Thus multivalent cations are especially harmful in destabilizing anionic latex. The ionic strength is one measure of the destabilizing effect of a solution on latex. The product of the salt concentration and the square of the ionic charge determine the ionic strength; therefore, equamolar amounts of Na+, Ca++, and Al+++ have relative effects of 1, 4, and 9 respectively. By using both different multivalent salts and different concentrations, one can devise increasingly more severe latex stability tests and establish different echelons of latex stability.
The effect of temperature is also substantial. As the temperature increases, eventually the higher kinetic energy of the latex particles may allow them to overcome the electrostatic repulsion, collide and coalesce. Consequently, a combination of high electrolyte concentrations of multivalent cations and elevated temperatures constitutes an especially severe set of conditions for latex stability. Indeed, commercial latexes are considered "excellent" if they can withstand the slow addition of 10 mL of 2% calcium chloride to about 50 mL of latex, even at room temperature. It is well known that as the temperature is increased then the stability of latex in the presence of salts is greatly reduced. For this reason, room temperature tests are used that call for much higher electrolyte concentrations than is actually encountered in an application so as to compensate for needing to function at higher temperatures. Also, adding a hot salt solution to hot latex is less convenient as a screening test.
In electrolyte stability testing, the amount of residue or grit that is generated when the latex is "shocked" by adding the salt solution is measured. Naturally, the identity of the salt and the strength of the salt solution determine the amount of residue produced. The rate of addition of the salt solution, stirring of the latex, etc. can also have an effect in discerning between borderline cases or similar stabilities. The amount of residue generated in the test is not to be confused with grit or residue that may be formed during the latex manufacturing. For this reason the latex is first filtered free of fine grit prior to testing.
It will be appreciated from the foregoing that latexes having high multivalent-ion stability may be useful in the processing and recovery of natural resources in the mining, petroleum and geothermal industries as well as in paper and textile coatings and construction mixtures employing substantial amounts of inorganic pigments or fillers.
For example, techniques for drilling and completing wells, particularly gas and oil wells, are well established. Of chief concern here are those wells which are drilled from the surface of the earth to some subterranean formation containing a fluid mineral which it is desired to recover. After the fluid containing geologic formation is located by investigation, a bore-hole is drilled through the overlying layers of the earth's crust to the fluid containing geologic formation in order to permit recovery of the fluid mineral contained therein. A casing is then positioned within the borehole to insure permanence of the borehole and to prevent entry into the well of a fluid from a formation other than the formation which is being tapped. This well casing is usually cemented in place by pumping a cement slurry downwardly through the well borehole, which is usually accomplished by means of conducting tubing within the well casing. The cement slurry flows out of the open lower end of the casing at the well bottom and then upwardly around the casing in the annular space between the outer wall of the casing and the wall of the well borehole.
Gas channeling is a phenomenon that occurs during the setting of the cement slurry. Once the cement slurry begins to set, the hydrostatic pressure in the cement column begins to decrease. This reduction in hydrostatic pressure allows the channeling of gas. This phenomenon occurs during setting of the cement, from the time when setting has progressed enough for the hydrostatic pressure to no longer be transmitted, or to no longer be sufficiently transmitted through the cement, but not enough for the cement at the level of the gas pocket to oppose migration of the gas into the setting cement under the pressure from the gas pocket which at this point is no longer balanced by the hydrostatic pressure.
The pressurized gas then migrates through the cement slurry in the course of its setting and/or between the cement and the drilled formations, creating a multiplicity of channels in the cement, which channels may reach up to the surface of the well. It will be appreciated that gas channeling can be exacerbated by the cement's shrinkage and possibly by liquid losses from the cement slurry through filtration into the surrounding earth, especially in the area of porous formations, also termed "fluid loss".
Gas channeling is thus a serious drawback leading to weakening of the cement and to safety problems on the surface. Various styrene-butadiene latexes have been used as an additive for oil and gas well cementing, primarily to control gas channeling. For example reference is made to U.S. Pat. Nos. 3,895,953; 3,043,790; 4,151,150 and 4,721,160, incorporated herein by reference. It will be appreciated that cements typically include calcium, aluminum, silicon, oxygen and/or sulfur and which set and harden by reaction with water. These include those cements commonly called "Portland cements" such as normal Portland or rapid-hardening or extra-rapid-hardening Portland cement, or sulfate-resisting cement and other modified Portland cements, cements commonly known as high-alumina cements, high-alumina calcium-aluminate cements. Although the latexes heretofore used have been found to function, further improved latexes are desired in systems containing alum, calcium carbonate, gypsum, zinc oxide, aluminum calcium phosphate, natural high-hardness brines, and other multivalent inorganic materials.
It is an object of the present invention to provide a polymeric latex with high multivalent-ion stability. It is another object of the present invention to provide a styrene butadiene based latex functionalized with a sulfonated acrylamide monomer that exhibits high tolerance to multivalent elctrolytes, even at elevated temperatures. Another object of the present invention is to provide a latex that may be useful in the processing and recovery of natural resources in the mining, petroleum and geothermal industries as well as in paper and textile coatings and construction mixtures employing susbstantial amounts of inorganic pigments or fillers. More particularly, it is an object of the present invention to provide a polymeric latex with high multivalent ion stability which is relatively inexpensive, and provides superior fluid loss control without adversely affecting other critical properties of the cement slurry for oil and gas well cementing. It is yet another object of the present invention to provide a polymeric latex useful as an additive for cement compositions for cementing wells. It has been discovered in accordance with the present invention, that a polymeric latex additive comprising styrene, butadiene and 2-acrylamido-2-methylpropanesulfonic acid when mixed with cement to form a slurry has the effect of limiting the porosity and blocking gas channeling. These and other objects and advantages will become more apparent from the following detailed description and examples.